In recent years, the widespread use of various types of cellular phones, mobile terminals, mobile computers, and car navigators has increased demands for lightweight, high-definition, high-brightness, and inexpensive small-size flat panel displays.
In households and offices, small-footprint desktop displays and flat panel displays such as wall-mounted televisions are replacing conventional CRT displays.
Particularly, with the proliferation of the high-speed Internet and the development of digital broadcasting, digital signal transmission at several hundred bits to several gigabits/sec has been put into practical use both in wired and wireless communication. Ordinary users are now beginning to exchange an extremely large amount of information in real time.
Under the circumstances, the flat panel displays require high-speed display operation suitable for digital signal processing in addition to a reduced weight, higher definition, higher brightness, and a lower cost than in conventional ones.
Liquid Crystal Displays (LDPs), Plasma Displays (PDPs), Field Emission Displays (FEDs) and the like are contemplated as the flat panel displays which meet the requirements. In addition to the various flat panel displays, attention is being focused on a new type of flat panel display, called an Organic Electroluminescence Device (OELD) or an Organic Light Emitted Diode (OLED).
The organic electroluminescence device achieves display by passing a current through an organic compound sandwiched between a cathode and an anode to cause light emission of fluorescent or phosphorescent organic molecules contained in the organic compound.
The studies of the organic electroluminescence device were conventionally centered on an organic semiconductor single crystal such as anthracene and perylene. In 1987, Tang et al. proposed a double-layered organic electroluminescence device formed by putting an emissive organic compound thin film on a hole-transport organic compound thin film to enable significant improvement in emission characteristics (an emission efficiency of 1.5 lm/W, a driving voltage of 10 V, and a brightness of 1000 cd/m2), which was a milestone in the studies.
Thereafter, element technologies were researched and developed such as dye-doping, a polymer OLED, an electrode with a low work function, and a mask deposition method. In 1997, an organic electroluminescence device employing a carrier injection method called a passive matrix method came into practical use. Development of an organic electroluminescence device with a new carrier injection method called an active matrix method has been considered.
Such an organic electroluminescence device is driven on the following principles. Specifically, a fluorescent or phosphorescent organic emission material is formed as a thin film between a pair of electrodes to inject electrons and holes thereinto from the positive and negative electrodes.
In the organic emission material, the injected electrode serves as a one-electron state of organic molecule (hereinafter referred to simply as an “electron”) entering the lowest unoccupied molecular orbital (LUMO) of the emissive molecule. The injected hole serves as a one-hole state of organic molecule (hereinafter referred to simply as a “hole”) entering the highest occupied molecular orbital (HOMO) of the emissive molecule. They move toward the opposite electrodes in the organic substance, respectively.
If the electron encounters the hole during the movement, a singlet or triplet excited state of the emissive molecule is produced and then deactivated while radiating light, thereby emitting light.
In general, many organic emission materials are known to have high quantum efficiency in response to optical pumping as in various types of laser dyes. To cause the materials to emit light by carrier injection, a high voltage on the order of several hundreds of volts was necessary in the initial organic electroluminescence devices since many organic compounds are insulators with poor carrier transport of electrons and holes. The abovementioned double-layered organic electroluminescence device by Tang et al. improved the emission characteristics by taking advantage of the excellent carrier transport of an organic electrophotographic photosensitive material used as a photosensitive material of a copier and by providing the two layers having different functions, that is, a thin film for transporting carriers (holes) and a thin film for emitting light. Today, a three-layered organic electroluminescence device has been reported in which a third organic thin film is responsible for electron transport of the other electric charge.
Besides, organic electroluminescence devices having multiple layers for individual functions have been proposed. They have additional thin films responsible for various functions such as a carrier injection layer for improving the injection characteristic of holes and electrons into an organic substance and a hole blocking layer for increasing the recombination probability of the holes and electrons.
All of the devices use, as the base of the light emission, the light radiation in the process of deactivation of the excited state from the organic emission molecules contained in the organic emission layer.
Many known organic emission materials which emit fluorescent or phosphorescent light have been developed for various applications such as inks, pigments, and scintillators. These organic emission materials are used in the organic electroluminescence devices.
The materials are broadly classified into a low molecular type and a high molecular type in terms of molecular weight. The low molecular type is formed into a thin film through a dry process such as a vapor deposition method, while the high molecular type is formed into a thin film through a cast method.
One of the reasons why the initial organic electroluminescence device before the improvement made by Tang could not realize high efficiency is said to be the inability to form a favorable organic thin film. The low molecular type, particularly, requires the following conditions: (1) the ability to form a thin film (on the order of 100 nm) with the vapor deposition method, (2) the ability to maintain a uniform thin film structure (without crystallization) after the film formation, (3) a high fluorescence quantum yield in a solid state, (4) appropriate carrier transport, (5) thermal resistance, (6) easiness of purification, and (7) electro-chemical stability.
In some cases, the organic emission materials may be classified, in terms of emission process, into an emission material which emits light directly from a recombination of an electron and a hole and a fluorescent material (or dopant material) which emits light through light pumping produced from an emission material.
From the viewpoint of differences in chemical structures, known materials are classified into a metal-complex type emission material (8-quinolinol, benzoxazol, azomethine, flavone or the like as a ligand; Al, Be, Zn, Ga, Eu, Pt or the like as a central metal), and a fluorochrome-type emission material (oxadiazole, pyrazoline, distyrylallylen, cyclopentadien, tetraphenylbutadien, bis-styrylanthacene, perylene, phenanthrene, olig-thiophen, pyrazoloquinoline, thiadiazopyridine, layered perovskite, p-sexiphenyl, Spiro compound or the like).
A wide variety of studies have been made for the emission materials of the organic electroluminescence device and the process for forming the device. However, much remains unclear about the efficiency which the organic electroluminescence device can achieve in emitting light.
The light energy taken out of the organic electroluminescence device is given by the number of emitting photons per electron or hole passing through the device. When this is expressed with the external quantum efficiency ηφ(ext) of electroluminescence, it is known that the following holds:ηφ(ext)=ηext×ηφ(int)=ηext×[γ×ηr×ηf]  (1)where ηφ(int) represents the internal quantum efficiency which indicates the number of emitting photons per electron or hole passing through the device within the device, ηext represents the out-coupling efficiency of light to the outside of the device after the light produced within the device is reduced by reflection and absorption at the interface of the device, γ represents the charge balance which corresponds to the ratio between the numbers of electrons and holes injected into the device, ηr represents the generation efficiency which indicates the proportion of singlet exciton generated from the injected carriers that contributes to emission, and ηf represents the emission quantum efficiency which indicates the proportion of emission and deactivation in the singlet exciton.
The external quantum efficiency ηφ(ext) corresponding to the amount of emission to the outside of the device can be broadly divided into ηr and ηf which are determined by the nature of the emission material itself, γ which is determined by the ratio between the electrons and holes injected into the device, and ηext which is determined by the structure of the device.
ηr and ηf are the efficiency relating to the property of the emission material itself and are determined uniquely by the emission material used. γ is the value determined by the electric potential difference and the interface potential between electrodes and an organic layer in contact therewith, the mobility of electrons and holes within the organic layer, and the like, and is determined uniquely by the properties of the electrode material and the organic substance within the device.
Of the factors, the charge balance γ is equal to or lower than one. The singlet exciton generation efficiency ηr is said to be equal to or lower than 0.25 in view of carrier spin. The emission quantum efficiency ηf is lower than one except in the super-radiative process. Thus, the part of the expression (1) determined by the organic substance within the device and the electrode material (the part [γ×ηr×ηf] in the expression (1)) is said to be equal to or lower than 0.25.
On the other hand, the out-coupling efficiency is determined by the laws of reflection and refraction of classical optics. Assuming that the refraction index of an emission layer is n, it is given by the following expression:ηext=1/(2n2)   (2)
The emission layers of many organic electroluminescence devices or glass substrates for holding them have a refractive index of approximately 1.6, and thus, ηext may be equal to 0.2. From the above values, the external quantum efficiency of electroluminescence ηφ(ext) is equal to or lower than 0.05(=0.2×0.25). In other words, the external quantum efficiency is 5% at most.
The factors which govern the efficiency of the organic electroluminescence device such as the charge balance, the exciton generation efficiency, the emission quantum efficiency, and the out-coupling efficiency can be enhanced to produce more light with less power. It can be expected that such improvement reduces the electric load on the device to extend the life thereof.
Several methods have been reported as means for extending the life of the organic electroluminescence device.
One of them is various means for protecting the electroluminescence device from outer moisture. For example, a light-curing resin can be applied to the outer surface of the device to provide protection from air.
An organic cover member and a glass material of a substrate can be surrounded by metal and brazed with appropriate metal having a lower melting point to prevent external air from entering from the end face of a sealing container.
An oxidation-resistant film made of aluminum or the like can be put on a cathode and serve as a second electrode to protect electrodes sandwiching an emission section.
Various means have been reported for providing a longer life through the use of an absorbent or improvement in element structure. For example, a moisture getter such as zeolite can be applied to the inside of a sealing container of an organic electroluminescence device.
In addition, heat transmission can be increased in a transparent substrate or a transparent substrate can be forcedly cooled. These approaches are provided to increase the life of a device by efficiently removing heat produced within the device.
Alternatively, various means have been reported for providing a longer life through increased durability of an organic layer material. For example, polyxylene can be used in a hole injection layer, a compound can be used which prevents crystallization in driving for a long time period, and a heat-resistant polyimide film can be used.
Also, a durable triphenylamine derivative can be used, and a durable diphenyl compound can be used.
In addition, an inorganic semiconductor can be used in a hole transport layer, and an anthracene derivative can be used.
In a disclosed example, adhesiveness between an organic layer and an electronic material can be enhanced to extend the life. For example, adhesiveness between an organic layer and an electrode can be improved by dispersing in the organic layer the same component as in the electrode in gradient concentration distribution.
A degradation preventing film which is not easily stripped such as polysilane can be interposed between ITO serving as an electrode and an emission layer.
Also, a higher efficiency and a longer life can be provided by reducing a voltage applied to electrodes. For example, a device with a longer life can be realized by stacking an inorganic amorphous hole injection layer, an inorganic electron-blocking layer, and an organic emission layer in order from an anode.
Corrosivity can be enhanced by coating the opposite side to a hole injection layer with metal having a high work function.
A longer life can be provided by effectively using an electric signal for driving. For example, a reverse bias can be applied after emission to prevent degradation.
The recombination probability of holes and electrons can be improved by increasing hole injection efficiency and providing an electron-blocking function in a hole transport layer to enhance electron transport on the cathode side than on the anode side.
A hole transport material can be dispersed in an emission layer or an electron transport layer to prevent degradation due to holes in the light emission layer or the electron transport layer, thereby extending the life.
O2 plasma treatment can be performed on the surface of ITO to extend the life.
A longer life can be provided by optimizing a substrate temperature during vapor deposition of an organic layer in the electroluminescence device.
A longer life can be achieved by optimizing a duty-ratio not by simple d.c. operation.
In addition, Patent Documents 1 and 2 below describe that the ratio of SnO2 to In2O3 of ITO serving as a transparent electrode is preferably 1 to 20% by mass, more preferably 5% to 12% by mass.    (Patent Document 1) JP-A-2001-223088    (Patent Document 2) JP-A-2001-223089
As described above, various methods have been proposed for providing a longer life.